1. Field of the Invention
The present invention relates to a method for erasing an image, in which the image is uniformly erased using a laser light and background fog on a thermoreversible recording medium caused by repetitive image erasure is reduced.
2. Description of the Related Art
Each image has been so far recorded and erased on a thermoreversible recording medium (hereinafter, may be referred to as “recording medium” or “medium”) by a contact method in which the thermoreversible recording medium is heated by making contact with a heat source. For the heat source, in the case of image recording, a thermal head is generally used, and in the case of image erasing, a heat roller, a ceramic heater or the like is generally used.
Such a contact image processing method has advantages in that when a thermoreversible recording medium is composed of a flexible material such as film and paper, an image can be uniformly recorded and erased by evenly pressing a heat source against the thermoreversible recording medium with use of a platen, and an image recording device and an image erasing device can be produced at cheap cost by using components of a conventional thermosensitive printer.
However, when a thermoreversible recording medium incorporates an RF-ID tag as described in Japanese Patent Application Laid-Open (JP-A) Nos. 2004.265247 and 2004-265249, the thickness of the thermoreversible recording medium is thickened and the flexibility thereof is degraded. Therefore, to uniformly press a heat source against the thermoreversible recording medium, it needs a high-pressure.
Moreover, in the contact type, a surface of the recording medium is scraped due to repetitive printing and erasure and irregularity is formed thereon, and some parts are not in contact with a heating source such as a thermal head or hot stamping. Thus, the recording medium may not be uniformly heated, causing decrease of image density or erasure failure. In particular, when erasure is performed at a low temperature in the range of the temperature at which an image can be erased, a part of the recording medium which is hard to come into contact with the heating source is not easily heated at the erasing temperature, causing erasure failure easily (Japanese Patent (JP-B) No. 3161199 and Japanese Patent Application Laid-Open (JP-A) No. 09-30118).
In view of the fact that RF-ID tag enables reading and rewriting of memory information from some distance away from a thermoreversible recording medium in a non-contact manner, a demand arises for thermoreversible recording media as well. The demand is that an image be rewritten on such a thermoreversible recording medium from some distance away from the thermoreversible recording medium. To respond to the demand, a method using a laser is proposed as a method of forming and erasing each image on a thermoreversible recording medium from some distance away from the thermoreversible recording medium when there are irregularities on the surface thereof (see JP-A No. 2000-136022).
It is the method by which non-contact recording is performed by using thermoreversible recording media on shipping containers used for physical distribution lines. Writing is performed by using a laser and erasing is performed by using a hot air, heated water, infrared heater, etc, but not by using a laser.
As such a recording method using a laser, a recording device (laser maker) is proposed of which a thermoreversible recording medium is irradiated with a high-power laser light to control the irradiation position. A thermoreversible recording medium is irradiated with a laser light using the laser marker, and a photothermal conversion material in the recording medium absorbs light so as to convert it into heat, which can record and erase the image. An image forming and erasing method using a laser has been proposed, wherein a recording medium including a leuco dye, a reversible developer and various photothermal conversion materials in combination is used, and recording is performed thereon using a near infrared laser light (see, JP-A Nos. 05-8537 and 11-151856).
However, occurrence of background fog is concerned in such thermoreversible recording medium (For example, see JP-B Nos. 3836901 and 3998193, and JP-A No. 2005-262798). Moreover, when repetitive erasure is performed on a thermoreversible recording medium using a high-output laser light, background fog occurs, causing decrease in contrast.
The decrease in contrast due to the background fog causes various problems such as trouble in reading barcode.
JP-B No. 3790485 proposes a solution to the background fog in which erasure is performed at a laser irradiation time shorter than that upon recording. However, when image processing is performed in a wide area of a thermoreversible recording medium, or when image processing is performed on a thermoreversible recording medium used for a shipping container which is employed in a physical distribution line in a non-contact manner, there exists problems, for example, an image is not sufficiently erased due to energy shortage of a laser light depending on a degradation state of the medium, a distance between the medium and an image recording device on which a laser light source is mounted, and a traveling speed of the thermoreversible recording medium in the line.
Thus, a method for controlling an energy to the thermoreversible recording medium only upon image erasure is necessary, in order to uniformly erase the image, and to obtain a clear contrast image by inhibiting occurrence of background fog.
JP-B No. 3161199 discloses an image erasing method in which an image is erased with an energy lower than the center value of the range of the energy which can erase the image on the thermoreversible recording material upon erasing the image, as an image erasure technique using a thermal head or hot stamping.
However, although the image erasure technique is applied to the thermoreversible recording medium containing a photothermal conversion material, on which an image can be erased by a laser light, the background fog cannot be sufficiently prevented.